Thermography has recently established itself alongside other methods for characterizing components such as, for example, ultrasound, magnetic-field and eddy-current methods, since, in contrast to the other methods, it works contactlessly and through imaging and can thereby achieve an increased measurement speed and an increased resolution and can be automated more easily.
A variety of approaches exist for determining quantitative parameters such as, for example, the geometry of a component or its thermal properties by means of thermography. Common to all thermographic measurement methods is their use of infrared radiation, which is radiated from the surface of a heated component in order to record a temporal progression of the surface temperature. A thermal imaging camera is frequently used for capturing the infrared radiation, in order to record a temporal progression of a planar thermal image.
To determine material parameters, frequently not only is the intrinsic heat of the component used, but, in addition, heat is introduced into the component so as selectively to induce a thermal flux. To heat the component, hitherto known methods include, among others, application of en electric current, irradiation with microwaves, use of chemical processes and use of radiation (lasers, halogen lamps, flash-lamps). Of these methods, the use of flash-lamps, in which light pulses are emitted by the flash-lamps which heat the component surface, is particularly widespread. The thermal flux generated in this manner is directed from the surface into the component. By analyzing the surface temperature, a parameter such as a wall thickness can be determined: the surface temperature decreases as long as the heat can flow away into the interior and reaches a constant level once the thermal front has reached the back of the component and the heat has consequently spread evenly throughout the component. The time needed to produce the thermal equilibrium and thus the temperature on the measured component surface is a measure of the component thickness (wall thickness).
Several methods for determining a wall thickness by means of flash thermography are known. In one method, the measured temperature signal is compared with a reference signal which is either measured, as disclosed in EP 1173724, or is calculated, as disclosed in EP 1203224. Alternatively, the time-dependent temperature signal can be transformed into the frequency domain, as known for example from EP 1203199 or Maldaque X. P., Marinetti S., “Pulse Phase Infrared Thermography”; J. Appl. Phys., 79(5) (1996), pages 2694-2698. WO 2001/41421 discloses a method for calculating the derivation of data recorded with an infrared detector in order to determine the wall thickness by means of the position of an extreme value in the derivation of the time-temperature signal.
However, use of a pulsed excitation, as occurs in flash-lamps, restricts the maximum measurable thickness: if the temperature difference on the surface falls below the noise level of the detector, the transient surface temperature can no longer be evaluated. If the object to be measured permits higher surface temperatures, multiple pulses can increase the heat supply, or the excitation period can be increased. As an alternative to this, a continuously modulated heat source can be used with suitable evaluation methods which, together with lock-in detection methods, can reduce the noise depending on the measurement period, as described, for example, in U.S. Pat. No. 4,878,116 and DE 4203272. WO 2000/11450 discloses a use of multiple frequencies at one time.
DE 4343076 C2 discloses a device for the thermal testing of a surface of, in particular, a moving object by means of thermography with optical excitation.
WO 2006/037359 A1 discloses a method for determining material parameters of an object from data of a temperature-versus-time plot.